Thin phosphorus nitride film as an n-type doping source used in a laser doping technology

ABSTRACT

An improved method and system for laser doping a semiconductor material is described. In the invention, phosphorous nitride is used as a dopant source. The phosphorous nitride is brought into close proximity with a region of the semiconductor to be doped. A pulse of laser light decomposes the phosphorous nitride and briefly melts the region of semiconductor to be doped to allow incorporation of dopant atoms from the phosphorous nitride into the semiconductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 09/473,576, filed Dec. 28,1999, now U.S. Pat. No. 6,586,318.

FIELD OF THE INVENTION

The invention relates to an improved method of laser doping asemiconductor material. In particular, the invention describes animproved doping source for use in laser doping.

BACKGROUND

In recent years, laser doping has become a popular method forfabricating large area electronics. The low processing temperatures usedin a laser doping process makes the process suitable for dopingsemiconductor layers on substrates that have low temperature tolerances.Examples of substrates with low temperature tolerances include plasticand glass.

A second reason for the popularity of laser doping is the availabilityof techniques to make self-aligned amorphous silicon Thin FilmTransistors (TFT). Self aligned techniques for forming TFTs utilize thegate electrode as a mask, thereby minimizing alignment problems whenforming a passivation island over the gate electrode. Detailed methodsfor forming such self aligned structure are described in an article byP. Mei, G. B. Anderson, J. B. Boyce, D. K. Fork, and R. Lujan entitledThin Film Transistor Technologies III published in the ElectrochemicalSoc. Proc. PV 96-23., p. 51 (1997). U.S. patent application Ser. No.D/97343 entitled “Method of manufacturing a Thin Film Transistor withreduced parasitic capacitance and reduced feed-through voltage” by P.Mei, R. Lujan, J. B. Boyce, C. Chua and M. Hack, also describesfabricating a TFT structure using a self alignment process and is herebyincorporated by reference.

During laser doping, a laser pulse briefly melts a surface layer in adoping region of a semiconductor. While the doping region is in a moltenstate, a dopant source is introduced near a surface of the dopingregion. The dopant source provides dopant atoms that are distributedthrough the surface layer. After the doping region solidifies, thedopants are available for electrical activation.

Typical dopant sources used to provide dopant atoms include: (1) gasdopant sources such as phosphorus fluoride (PF₅) and boron fluoride(BF₃), (2) spin-on dopants such as phosphorous doped silica solution,and (3) phosphor-silicon alloys which may be ablated from anothersubstrate or deposited directly on the doping region.

Each of the above described dopant sources has a correspondingdisadvantage. For example, gas dopant sources are difficult to controland require the use of specialized equipment to prevent unevendistribution of dopants. Furthermore, both gas and spin-on depositiontechniques have low doping efficiencies. A low doping efficiency makesit difficult to fabricate a heavily doped material. Phosphor-siliconalloy materials are difficult to deposit and unstable when exposed tomoisture in air. In particular, moisture in the air decomposes thephosphor-silicon alloy. After decomposition, the moisture mayredistribute the dopant atoms. Dopant atom redistribution produces highdefect rates in devices formed from the doped semiconductor.

Ion implantation provides an alternate method of distributing dopantatoms in a semiconductor. However, when used in large area processes,ion implantation requires expensive specialized equipment.

Thus an improved method of laser doping is needed.

SUMMARY OF THE INVENTION

Although laser doping has become a popular method of fabricating largearea electronic devices, current laser dopant sources are difficult tocontrol, require expensive equipment, or are unstable when exposed tomoisture in the air. In order to avoid these problems, the inventiondescribes a technique for using a thin film of dopant containingcompound (DCC) as a laser doping source. The dopant containing compound(DCC) can be deposited by standard equipment and is stable in air. Anexample of a dopant containing compound is phosphorous nitride.

In one embodiment of the invention, standard plasma enhanced chemicalvapor deposition (PECVD) equipment is used to deposit a thin phosphorousnitride film over the dopant region. Alternatively, a transparent dopantplate may be used to position the phosphorous nitride near the dopantregion in a laser ablation process. In laser ablation, a laserdecomposes the phosphorous nitride. The laser also briefly melts thesemiconductor in the dopant region to allow incorporation of dopantatoms into the doping region. After the doping process, remainingphosphor nitride may be removed using a plasma containing small amountsof CF4 diluted in oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sequence of operations used to fabricate a ThinFilm Transistor (TFT) structure using one embodiment of the invention.

FIG. 2. is a graph illustrating the resistivity of laser doped amorphoussilicon as a function of changing laser energies and dopant sources.

FIG. 3 illustrates a system that uses laser ablation to provide dopantatoms from a film of phosphorous nitride to dope a semiconductor sample.

FIG. 4 is a plot illustrating the TFT transfer characteristic of adevice fabricated using a phosphorous nitride dopant.

DETAILED DESCRIPTION

FIG. 1 illustrates the fabrication of a thin film transistor (TFT) bylaser doping portions of the TFT with a phosphorous nitride thin film.Although the structure described is a TFT structure, it is understoodthat the described doping techniques are applicable to a large range ofdevices including other semiconductor active and passive devices such asresistors and transistors used in integrated circuits.

In FIG. 1, a metal gate 104 is deposited and patterned over a glasssubstrate 108. A deposition techniques, such as plasma enhanced chemicalvapor deposition (PECVD) is used to deposit a plurality of layers overthe gate 104 and substrate 108. In the embodiment illustrated, thelayers include a nitride layer 112, an amorphous silicon layer 116 and atop passivation layer 120. In one embodiment of the invention, toppassivation layer 120 is a multilayer structure formed from alternatingnitride and oxide multilayers. A backside exposure that uses metal gate104 as a mask may be used to form an island 124 from the top passivationlayer 120. Using a backside exposure simplifies the alignment of island124 with gate 104. The procedures used to form and pattern the gatemetal, to deposit the nitride, amorphous silicon and passivation layersand to pattern the passivation layer are described in the previouslyreferenced patent application entitled “Method of Manufacturing a ThinFilm Transistor with Reduced Parasitic Capacitance and ReducedFeed-Through Voltage”.

After formation of island 124, a doping region of amorphous silicon isdoped. In the illustrated embodiment, doping regions 129 are positionedon both sides of island 124. To dope the amorphous silicon, a DCCcompound that includes at least one element from either from column IIIor column V of the periodic table is brought in close proximity to theamorphous silicon. Typical compounds may include the column III orcolumn V element chemically bonded to form a nitride or oxide. As usedherein, a “compound” is defined as a combination of two or more elementsthat form a chemical bond. Thus a “compound” does not include mixturesor alloys in which the elements are not chemically bonded together. Forexample, water is defined as a compound whereas a mixture of hydrogenand oxygen is for purposes of this patent not considered a compound.

One example of a compound that has been found to be suitable as a dopingsource is phosphorous nitride. The chemical bond between phosphor andnitride in the phosphor nitride compound makes the compound stable inair; phosphor itself is unstable in air.

A first method of forming phosphorous nitride is to evaporate phosphorin a N₂ plasma. The evaporated phosphor deposits over the substrate as aphosphorous nitride film. Alternatively, a pulse discharge of aphosphorus powder covered electrode in a N₂ ambient gas may also be usedto form a phosphorous nitride compound. Both techniques of generating aphosphorous nitride film are described in an article entitled “SolidPlanar Diffusion Sources Based on Phosphorus Nitride Prepared by aOne-stage Process in a Pulse Discharge” by M. Raicis and L. Raicis inSurface and Coatings Technology, Vol. 78, p. 238 (1996).

An alternate method of bringing a DCC, including an element from column3 or column 5 of the periodic table, into close proximity to amorphoussilicon uses commercially available PECVD equipment. In the followingdescription, example parameters used to achieve PECVD deposition of aphosphorous nitride film 130 will be provided. In the example, aprocessing gas of one sccm (Standard cubic centimeter per minute) PH₅and 100 sccm NH₃ is formed. Ten watts of 13.5 MHz radio frequency powerin a 400 mTorr ambient pressure converts the processing gas into aplasma. Exposing a substrate, such as glass substrate 108 at 250 degreesCentigrade, to the plasma results in a phosphorous nitride thin filmwith a reflection index of approximately 1.85. Using the providedexample parameters results in a deposition rate of approximately 50Angstroms per minute.

After deposition, a laser beam 132 irradiates the phosphorous nitridethin film to laser dope select regions of amorphous silicon 116. In oneembodiment, laser beam 132 originates from an excimer laser with a highfluence laser beam having a wavelength of 308 nm. The laser beam brieflymelts the amorphous silicon and decomposes the phosphorous thin film128. The decomposing phosphorous thin film introduces phosphor atomsinto the molten layer of amorphous silicon.

FIG. 2 is a graph that illustrates the sheet resistance of a dopedamorphous silicon layer as a function of laser energy density used inlaser doping. Each curve of the graph corresponds to a different dopantsource. Curve 204 corresponds to a phosphorous nitride dopant source,curve 208 corresponds to a Psi alloy as a dopant source, curve 212corresponds to a P+ implantation at a dosage of 3×10¹⁵ cm⁻² and curve216 corresponds to a P+ implantation at a dosage of 1×10¹⁵ cm⁻². Themeasurements were taken on a structure including a nitride and a 500angstrom thick amorphous silicon layer deposited on a glass substrate.

To form the structure from which measurements were taken, 100 Angstromsof phosphorous nitride film was deposited over the amorphous siliconusing the previously described example PECVD process. Pulses of laserlight from a XeCl excimer laser decomposes the phosphorous nitride.

After exposure to the excimer laser, a four point probe was used tomeasure the resistivity of the resulting doped amorphous siliconstructure. At each laser energy density value plotted along a horizontalaxis of FIG. 2, the phosphorous nitride dopant source plotted as curve204, produced the lowest resistance doped amorphous silicon. A lowresistance demonstrates the high doping efficiency of the phosphorousnitride by indicating that almost all of the phosphorous nitride filmdecomposes under the laser exposure.

Returning to FIG. 1, after exposure of portions of the phosphorousnitride thin film to laser radiation, areas unexposed to laser radiationmay remain covered by the phosphorous nitride film. In particular,during fabrication of laser-doped, self-aligned TFT structures, portionsof phosphorous nitride films 128 deposited over passivation island 124may not absorb or receive sufficient exposure to laser energy and thusmay not be fully decomposed. Unexposed phosphorous nitride film has avery high resistivity. Therefore, in some TFT embodiments, the unexposedphosphorous nitride remains in the device without disturbing theself-aligned TFT operation. The high resistivity of phosphorus nitrideallows its use as a gate dielectric material for InP MISFETS asdescribed in an article entitled “Enhancement mode InP MISFET's withsulfide passivation and photo-CVD grown P3N5 gate insulators” by Y.Jeong, S. Jo, B. Lee and T. Sugano in the IEEE Electron Device Letters,Vol 16, Page 109 (1995).

When complete removal of phosphorous nitride is desired, a plasmacleaning process may be implemented. In one implementation of such aplasma cleaning process, a small amount of CF4, typically 1 to 2 percentin O₂, is used to remove the undesirable phosphorous nitride.

After removal of undesired portions of phosphorous nitride thin film128, metal source contact 136 and metal drain contact 140 are formedover the doped areas of amorphous silicon layer 116. To complete the TFTstructure, a second passivation island 144 of SiO₂ may be formed betweenmetal source contact 136 and metal drain contact 140.

A typical transfer characteristic curve of a TFT fabricated by theprocess described above is shown in FIG. 4. FIG. 4 plots a normalizeddrain current as a function of an applied gate voltage. The performanceis comparable to devices formed using dopant sources that are unstablein air such as structures described in the previously referenced patentby Mei et. al.

Although the prior description has described using a PECVD technique todeposit the phosphorous nitride directly on a device for laser doping,it should be recognized that other techniques may be used to bringphosphorous nitride in close proximity to the device to be doped. Forexample, FIG. 3 shows a system 300 for depositing phosphorous nitrideusing laser ablation. In FIG. 3, phosphorous nitride film 302 isdeposited over a surface of a transparent source plate 304. Thetransparent source plate is positioned in close proximity to thesemiconductor sample 306 with the phosphorous nitride film coatedsurface facing semiconductor sample 306. Gap 312 that separates thephosphorous nitride film 302 and semiconductor sample 306 is typicallyseveral microns, although the gap may range from zero to severalmillimeters. The height of spacers 308 determines the size of the gap.The smaller the gap between phosphorous nitride film 302 andsemiconductor sample 306, the higher the dopant efficiency.

After phosphorous nitride film 302 is properly positioned over thesemiconductor to be doped, a laser such as an excimer laser, outputs alaser beam that propagates through source plate 304 and irradiates aselect area of phosphorous nitride film 302. The phosphorous nitridefilm absorbs a portion of the laser energy. When the phosphorous nitridefilm absorption rate is too low, a thin layer of opaque material, suchas a-Si, may be inserted between plate 304 and phosphorous nitride film302 to enhance the efficiency of laser energy absorption. Absorbedenergy from the laser pulse ablates phosphorous nitride film 302 causingphosphorous nitride film 302 to release energetic dopants into the gapbetween film 302 and semiconductor sample 306.

In addition to ablating phosphorous nitride film 302, the laser energyalso melts a surface region of semiconductor sample 306. The depth ofthe melted region depends upon the laser energy and the pulse width(duration) as well as the thermal transport properties of thesemiconductor (typically amorphous silicon) sample 306. A typical laserpulse width is approximately 50 nanoseconds. After receiving the laserpulse, the semiconductor remains in a molten state for a short timeinterval allowing semiconductor sample 306 to incorporate some of thedopant atoms released in the gap during the laser ablation process. Theabsorbed dopants are activated when semiconductor sample 306 solidifies.A more detailed description of using laser ablation is described in U.S.Pat. No. 5,871,826 (the '826 reference) entitled Proximity Laser DopingTechnique for Electronic Materials by inventors Ping Mei, Rene A. Lujanand James B. Boyce which is hereby incorporated by reference.

In the '826 reference, a PSi doping material is the suggested dopingmaterial. However, PSi doping material tends to break down when exposedto moisture in air. The stability in air of DCC compounds, such asphosphorous nitride, allows the dopant source plate to be prepared offline in a batch process and stored until needed for use.

While the invention has been described in terms of a number of specificembodiments, it will be evident to those skilled in the art that manyalternative, modifications, and variations are within the scope of theteachings contained herein. For example, variations in parameters mayoccur, such as the type of DCC compound, the method of deposition of theDCC compound, the type of laser used and the device being formed.Examples of other types of devices that may be formed using laser dopingof phosphorous nitride include the fabrication of shallow junctions inVery Large Scale Integrated Circuits (VLSIs). Accordingly, the presentinvention should not be limited by the embodiments used to exemplify it,but rather should be considered to be within the spirit and scope of thefollowing claims, and equivalents thereto, including all suchalternatives, modifications, and variations.

What is claimed is:
 1. A method of forming a thin film transistorstructure comprising the operation of: depositing a gate on a substrate;forming an amorphous silicon layer over the gate and the substrate;forming a passivation layer over the amorphous silicon layer; creatingan island from the passivation layer over the gate; positioning dopantcontaining compound over the amorphous silicon layer; exposing thedopant containing compound to a laser beam to dope the amorphous siliconwith atoms from the dopant containing compound to create doped regionsof amorphous silicon.
 2. The method of claim 1 further comprising:depositing and patterning metal contacts for a source and drain over thedoped regions of amorphous silicon.
 3. The method of claim 1 wherein thecreating of the island from the passivation layer utilizes a backsideexposure in which the gate serves as a mask.
 4. The method of claim 1further comprising: removing excess dopant containing compound usingplasma of dilute CF4 in oxygen.
 5. The method of claim 1 wherein thedopant containing compound is phosphorous nitride.
 6. The method ofclaim 5 wherein the positioning of the phosphorous nitride is done usinga plasma enhanced chemical vapor deposition process.
 7. The method ofclaim 5 wherein the positioning of the phosphorous nitride is done bypositioning a transparent source plate, the transparent source plateincludes a film of phosphorous nitride.
 8. The method of claim 5 whereina gap separates the phosphorous nitride film from the amorphous silicon,the doping of the amorphous silicon occurs through phosphor and nitrogenatoms that cross the gap.